Remove GET v3/roles/{role_id}/users and add GET v3/role_assignments

Remove non-existent operation (GET v3/roles/{role_id}/users
Add missing GET v3/role_assignments

Closes-Bug: #1284895

Change-Id: Iffbc6325bf9a32a21662a2bad256ed9f55689460
author: diane fleming
Merge "Remove GET v3/roles/{role_id}/users and add GET v3/role_assignments"